1. Field of the Invention
The present invention relates to a technique for recording/reproducing information on/from a recording medium using a plurality of probes, for example, to a high-density data recording/reproduction technique adopting the STM principle.
2. Related Background Art
As conventional memories, semiconductor memories and magnetic memories using magnetic materials and semiconductors materials are popular. In recent years, along with development of the laser technique, inexpensive, high-density recording media comprising optical memories using organic thin films such as organic dyes, photopolymers, and the like are available.
Recently, a scanning tunnel microscope (to be abbreviated as an STM hereinafter), which can directly observe an electron structure of atoms on the surface of a conductor, has been developed [B. Binning et al. Phys. Rev. Lett. 49.57 (1982)]. The STM allows a high-resolution measurement of a real space image regardless of mono-crystalline or amorphous structures. In addition, the STM allows to observe a sample with a low electrical power without damaging the sample by a current. Furthermore, since the STM can operate in air, and can be used for various materials, a wide application range is expected. The STM utilizes a phenomenon that when a voltage is applied between a metal probe (probe electrode) and a conductive substance, and the probe electrode is brought close to the conductive substance, i.e., a position at a distance of about 1 nm therefrom, a tunnel current flows. This current is very sensitive to a change in distance between the probe electrode and the sample. When the probe electrode is scanned to maintain a constant tunnel current, various kinds of information associated with all the electron clouds in a real space can be read. In this case, the resolution in the planar direction (in-surface direction) is about 0.1 nm.
Therefore, high-density recording/reproduction on the atomic order (sub nanometer order) can be satisfactorily performed by applying the STM principle. For example, in a recording/reproduction apparatus disclosed in Japanese Patent Application Laid-Open No. 61-80536, data is written by removing atomic particles attached to the surface of a medium by, e.g., an electron beam, and the written data is reproduced by the STM.
Japanese Patent Application Laid-Open Nos. 63-161552, 63-161553, and the like disclose a method of recording/reproducing information by the STM using, as a recording layer, a thin film layer of a material such as a K electron-based organic compound or a chalcogen compound having a memory effect with respect to voltage-current switching characteristics. With this method, if the recording bit size is assumed to be 10 nm, large-capacity recording/reproduction of 10.sup.12 bits/cm.sup.2 can be realized.
For the purpose of attaining a further compact structure, Japanese Patent Application Laid-Open Nos. 62-281138, 1-196751, and the like propose an apparatus wherein a plurality of probe electrodes are formed on a semiconductor substrate, and recording is performed by rotating a recording medium opposing the semiconductor substrate. For example, when a multi-probe head having 2,500 probe electrodes arranged on a 1-cm.sup.2 silicon chip is combined with a material having a memory effect, large-capacity digital data recording/reproduction having a recording capacity of 400 Mbits per probe and a total recording capacity of 1 Tbit can be realized.
When a recording/reproduction apparatus using a multi-probe head is constituted in practice, the following points must be taken into consideration.
(1) It is difficult to manufacture 2,500 probe electrodes constituting the multi-probe head without defects. Therefore, in order to increase the manufacturing yield, defective probe electrodes must be permitted to some extent. PA1 (2) The probe electrodes may suffer from some damage after they are repetitively used. When one probe electrode is damaged, 400-Mbit data is lost. In order to assure reliability, recording data must be restored even when the probe electrodes are damaged. PA1 (3) A recording medium is required to have evenness on the molecular order. Even when a recording layer having high orientation characteristics is formed by utilizing, e.g., the Langmuir-Blodgett technique, it is very difficult to completely remove pinholes on the molecular order. Therefore, a countermeasure against data omission caused by a partial defect of a recording medium must be taken. PA1 (1) In order to provide redundancy for a probe defect, it is possible to arrange redundant probe electrodes. However, since the number of probe electrodes is as large as 2,500, a mechanism for detecting defective probe electrodes, storing the positions of the defective probe electrodes, and replacing the defective probe electrodes with redundant probe electrodes is complicated. PA1 (2) Damage to the probe electrodes and partial defects of a recording medium result in burst errors, and the above-mentioned error correction method can be applied in principle. However, damage of one probe electrode corresponds to a 400-Mbit burst error, and a 100-.mu.m.sup.2 medium defect corresponds to a 100-Mbit burst error. In order to perform error correction corresponding to such a large burst error length, a large encoding calculation amount, and a large work memory space are required. This is because a series of code lengths must be set to be considerably longer than a burst error length even when double-encoding is performed. For this reason, the apparatus becomes large in size, and large-capacity (10.sup.12 bits/cm.sup.2), compact, and high-density characteristics cannot be utilized.
In conventional semiconductor memories, in order to improve reliability, a redundancy bit is prepared in advance, and when a defective bit is generated, the defective bit is replaced with the redundancy bit, thus compensating for the defective bit generated in a semiconductor manufacturing process. On the other hand, magnetic disks, optical disks, and the like employ an error correction method to compensate for a partial defect of a medium. In a recording mode using the error correction method, a k-bit input signal is converted into an n-bit code, and the n-bit code is recorded on a medium. In a reproduction mode, an n-bit signal is read from the medium, is decoded to a k-bit signal, and the k-bit signal is output. In this case, k&lt;n, and even when some bits of the n-bit encoding word recorded on the medium change due to an error, an original correct signal can be decoded in a decoding mode. A large number of encoding methods such as BCH encoding, Fire encoding, Reed-Solomon encoding, and the like have been proposed according to applications. The error correction based on encoding can mainly remove a random error. On the other hand, a burst error caused by scratches on a recording medium, peeling of a recording layer, or the like is coped with by combining an encoding as described above and an interleave method. In this case, an encoding word string must be double-coded using product codes or continuous codes over a data area sufficiently larger than an expected maximum burst error length.
Note that in the interleave method, a burst error is distributed to some encoding words to be converted to short burst errors or random errors, thereby performing error detection and correction. For example, a k-bit input signal is encoded to n bits (such encoding will be referred to as (n,k) encoding hereinafter). L n-bit encoding words are collected, and are arranged on an L (columns).times.n (rows) matrix, as shown in FIG. 1. The encoding words are sequentially output from the first row to obtain new L-bit encoding words. The L-bit encoding words are recorded. Thus, a burst error of L bits or less can be distributed to L recorded signal words bit by bit. In other words, a burst error is converted into random errors.
However, in a memory device for performing recording/reproduction on the molecular order using 2,500 multi-probe electrodes, the following subjects must be attained to assure reliability against damage to the probes, partial defects of a medium, and the like using the above-mentioned conventional error correction method.